Brazing filler metals are metal alloys suitable for soldering and having working temperatures of greater than 450.degree. C. They are frequently composed of silver with added copper, zinc and cadmium.
Applicants are not aware of any universal brazing filler metal that are, in practice, suitable for use with fluxing agents or under vacuum or inert gas whereof the working temperature is under 600.degree. C. But the lower the working temperature, the lesser is the extent to which the mechanical properties of the workpiece to be brazed are impaired during the brazing process by heating to the brazing temperature. Apart from the influence on the strength, particularly of quenched and drawn steel, the thermal stresses also remain less when the working temperature of the brazing filler metal is kept low. At the same time, the energy requirement for the brazing process is reduced. It is therefore important to have available brazing filler metals with working temperatures that are as low as possible.
Brazing filler metals having working temperatures of between 600 and 700.degree. C. are composed predominantly of alloys of the metals silver, copper, tin, zinc and cadmium. But it is known that cadmium and its volatile oxides have a toxic effect when they are taken up into the human body. This incorporation cannot always be completely eliminated when brazing is performed incorrectly with cadmium-containing brazing alloys, so that there is the danger that symptoms of poisoning may appear. This gives rise to the requirement to greatly reduce the cadmium content in brazing alloys or, where it is technically feasible, wholly to avoid it. This is moreover highly desirable on general ecological grounds.
In the majority of the brazing filler metals used hitherto, which have working temperatures of between 600 and 700.degree. C., a more or less high cadmium content is necessary in order to attain these low temperatures. The lowest-melting cadmium-containing solder used hitherto has the composition Ag 44, Cu 19, Zn 21, Cd 20 and a working temperature of 610.degree. C. It results moreover in the brazed joint produced having outstanding mechanical properties. The brazed joint has high tensile strength and is very ductile. The ductility is frequently expressed by so-called bending angles which are measured on edgeless brazed joints. Measurement of the bending angle relies on the measurement of welded samples according to DIN 50 121. Two steel rods with dimensions of 4.times.4.times.40 mm are butt-welded together. The sample is then bent until a tear occurs in the brazed area. The measured angle is called the bending angle.
Cadmium-free brazed alloys have been in existence for a long time and are being used with increasing frequency. The cadmium-free hard solders known to date are conventionally composed of silver, copper, zinc and tin. The working temperatures of these solders are however from 80 to 120.degree. C. above those of the corresponding cadmium-containing solders, which is unacceptable in temperature-sensitive workpieces, so that even today cadmium-containing solders are still necessary in many cases. It is believed that the lowest-melting cadmium-free hard solder has the composition Ag 56, Cu 22, Zn 17, Sn 5 and a working temperature of 650.degree. C. Brazed joints with this filler metal do indeed show relatively high strength and ductility, nevertheless the ductile properties in particular, that is, the bending angles, of soldered joints with cadmium-containing hard solders are not achieved. There certainly also exist brazing filler metals which achieve the ductility of soldered joints produced by cadmium-containing solders, but these solders have an even higher working temperature in excess of 700.degree. C.
From DE-AS 24 17 060 a cadmium-free hard solder alloy based on silver is known which contains from 40 to 50% by weight of silver, 15 to 38% by weight of copper, 22 to 32% by weight of zinc, 1 to 6% by weight of tin and 0.5 to 3% by weight of indium. The working temperatures of these brazing filler metals are between 710 and 630.degree. C., which is still too high for many applications. Furthermore, these alloys do not achieve the ductility required for many soldered joints.
Stratified contact-pieces for weak current contacts wherein the layer of solder is composed of a silver-copper-gallium alloy are described in DE-OS 33 15 498. They contain 60 to 75% by weight of silver, 18 to 35% by weight of copper and 5 to 8% by weight of gallium, with the latter component also being replaceable by 4 to 7% by weight of gallium plus 1 to 4% by weight of indium, or by 1 to 4% by weight of gallium and 3 to 7% by weight of tin. Melting points for these brazing alloys are not given. They are however above 650.degree. C.
Brazing alloys having 50 to 65% by weight of silver, 5 to 41% by weight of copper, 3 to 12% by weight of gallium and 6 to 18% by weight of indium are known from SU-PS 450 673 (Derwent Abstract 75-65066W7/39). Their melting points are between 640 and 680.degree. C. The alloys are moreover very brittle.
Brazing filler metals alloys containing 50 to 70% by weight of silver, 15 to 30% by weight of copper, 8 to 20% by weight of zinc and 0.1 to 8% by weight of gallium and/or indium are described in DE-OS 27 45 409. Their melting points are between 650 and 680.degree. C. and thus to some extent distinctly above those of cadmium-containing alloys.